The shell is built and cool, and it is still sitting on the master. Between here and part 3 there is exactly one step — cutting the release seam — and it has a whole page to itself for two reasons: it is the step where the skull gets ruined, and it is the step where people cut themselves.
Five minutes of inspection now saves an hour of patching later.
Before you cut: check the shell
Run your fingers over the whole surface with the shell still on the master. You are looking for three things.
- Thin patches. Press gently with a thumb. Anywhere that flexes noticeably is too thin and will tear when the shell is prised off. Add glue now, while the master is still inside to support it. The usual thin spots are the crown, the cheekbones and the brow ridge — the high points your nozzle skated over.
- Gaps. Hold the skull up to a lamp. Light coming through means bare foil, and bare foil is a hole waiting to happen. Patch it.
- The base ridge. Check it is continuous all the way round with no breaks. This is the part that carries the finished skull's weight, and a gap in it will show up as a collapsed jaw line three weeks later.
Let any patches cool fully before going on.
Where the seam goes
The shell has to be cut somewhere to get it off. Where you put that cut determines how much work part 3 will be.
The standard seam runs from the back edge of the base up to the crown, straight up the rear centreline. It is the right choice almost every time:
- The back of the skull is the flattest, least detailed area, so the closed seam has nothing to disturb.
- Almost every display angle has the back of the skull facing away from the viewer.
- It is the shortest path from the open base to the widest part of the cranium, which is what has to spread for the master to come out.
Two things to avoid absolutely. Do not run the cut across the top of the cranium from side to side — that seam sits at the most visible point and is unforgiving. And do not cut anywhere near the eye sockets or the nasal opening; those areas have the most relief, and a seam through them will never close cleanly.
Mark the line with a marker before picking up a blade. A cut placed by eye tends to wander off the centreline and end up on the side of the head.
Cutting it
Use a fresh hobby blade or a single-edged razor. A blunt blade needs force, force makes the blade skid, and a skidding blade on a curved skull goes somewhere neither you nor the skull wants.
The method:
- Put the skull down. Do not hold it in your lap or in your hand. Set it on the bench, base towards you, so both hands are free and neither is behind the blade.
- Score, do not slice. Make several light passes along the marked line rather than one deep cut. Each pass goes a little deeper. This is slower, and it is the difference between a controlled cut and a slip.
- Cut through the glue and the foil together, from the base edge upward to the crown, stopping at the top of the cranium. The cut does not need to go beyond that point — the shell will spread the rest of the way.
- Cut away from your body throughout. If the blade needs to travel towards you, turn the skull round instead.
This project is not worth losing a finger over, and it is genuinely the step in the archive where that is a real possibility. See the tools and safety page for blade handling, and consider a cut-resistant glove on the hand steadying the work.
When it goes wrong
- The blade wandered off the line. Not a problem. Follow the new line rather than trying to correct back — a single wandering seam closes as easily as a straight one, whereas a Y-shaped seam does not.
- The shell cracked away from the cut. The glue was too cold, or the shell is too thin there. Stop cutting, add glue to thicken the area, let it cool, and re-cut.
- The blade will not go through. The shell is thicker than you thought, which is a good problem. Change the blade and take more passes.
- You cut into the master. Only a nuisance if you were planning more copies. Foil hides a surprising amount of damage on the next go.
Once the seam is cut
Stop here and let everything reach room temperature before prising anything apart. The shell wants to be fully rigid when it is spread, not merely firm.
Continue to part 2b — releasing the shell and closing the seam · Back to part 1